Actually 12 ga is good for 25a (310.16)240.4(D) reduces that to 20a because the "small conductors" are most likely to serve receptacles where the installer has no control over what the user will plug in. They will keep plugging things in until the breaker trips, then unplug the clock. 240.4(D) forces the 80% rule on the user. Perhaps Europeans are more dilligent about load management.